This invention relates to a connecting structure of double pipes for carrying fluid, which is used mainly in chemical plant or semiconductor manufacturing plant.
In recent years, the integrating degree of an integrated circuit (IC) is being increased from 4M (Mega bit) to 16M and 64M. In its manufacturing process, the yield of the IC depends on how they can be free from a contaminated substance such as metal ion, total organic carbon (TOC) or the like that may occur from a piping where an ultrapure water is flowing.
The content of such contaminated substance in the ultra-pure water is now discussed in an order of PPB (parts per billion). Conventionally, the foregoing piping is made of a stainless steel, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyether keton (PEEK), polyphenylene sulfide (PPS) or the like, but such material is not suitable for manufacturing ULSI having more than 4M, because the ultra-pure water is contaminated by such a substance as metal ion, TOC or the like. In lieu of the foregoing material, the piping may be made of tetrafluoroethylene (TFE) or copolymer of TFE and perfluoroalkylvinylether (PFA) that is a chemically most stable resin suitable for manufacturing the semiconductors.
When using a piping made of TFE or copolymer of TFE and PFA, the problem is that it has a low rigidity, a joint difficulty in heat fusion, a joint inability by adhesive, a costly price and like. For this reason, there are some unusual cases, in which an internal surface of a metal double pipe is treated with a lining of 1 or 5 mm which is made of TFE or copolymer of TFE and PFA. However, respective flanges of both lining-treated double pipes are fixed with each other by screw clamping, so that they are not suitable for the piping for carrying an ultra-pure water in the semiconductor manufacturing plant. Because it is easy that some germs may occur in a joint portion of the both double pipes. Further, the thermal expansion coefficient of metal differs greatly from that of such resinous lining. When heating and cooling are repeated frequently one after another, the lining may cause some creases or cracking due to expansion or shrinkage. Accordingly, it is not suitable for the semiconductor production to use such lining-treated double pipe.